Fiber-to-fiber platform for multi-layer ferroelectric on insulator waveguide devices

ABSTRACT

A fiber-to-fiber system for multi-layer ferroelectric on insulator waveguide devices is described. The system comprises a fiber-to-chip coupler that couples light from a standard optical fiber to multi-layer ferroelectric on insulator waveguides. The multi-layer ferroelectric on insulator waveguides are integrated with electrodes to implement an optical device, an electro-optical device, or a non-linear optical device, such as an electro-optical modulator, with microwave and optical waveguide crossings compatible with packaging. A second fiber-to-chip coupler outputs the light from the multi-layer ferroelectric on insulator device to a standard optical fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication No. 63/026,968, filed on May 19, 2020, and entitled“FIBER-TO-FIBER PLATFORM FOR MULTI-LAYER FERROELECTRIC ON INSULATORWAVEGUIDE DEVICES,” the disclosure of which is expressly incorporatedherein by reference in its entirety.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Award No. 1809894awarded by National Science Foundation. The government has certainrights in the invention.

BACKGROUND

Ferroelectrics such as lithium niobate are workhorse materials formodern optical technologies. Ion-sliced lithium niobate technology hasemerged as an enabling platform for electro-optical modulators withhalf-wave voltage (V_(π)) in the range of 5 V and electrical bandwidthin the range of 100 GHz. Traditional bulk lithium niobateelectro-optical modulators are based on diffused titanium opticalwaveguides and microwave electrodes separated from the optical waveguideby a silicon dioxide buffer layer, illustrated in FIG. 1A. The approachcan produce 3.5 V V_(π) in a 35 GHz bandwidth. In contrast, emergingapproaches utilize optical waveguides based on thin films of lithiumniobate on insulator (LNOI). In FIG. 1B, the optical waveguide is formedby strip loading the thin film of lithium niobate with a high indexmaterial such as silicon nitride or titanium dioxide. The approach canenable modulators with 6.8 V V_(π) in a 100 GHz bandwidth. A thirdapproach, shown in FIG. 1C, involves etching the thin film of lithiumniobate to form ridge waveguides. The silicon dioxide buffer layer isremoved and the metal electrodes are placed on the same layer as thelithium niobate. The approach has experimentally achieved 4.4 V V_(π)with a 3 dB bandwidth of 100 GHz in a device length of 5 mm,representing the current state-of-the-art. High optical confinement inthe rib waveguide allows for small signal-to-ground gap spacing in themicrowave co-planar waveguide geometry. The small spacing allows forstrong electro-optical overlap between the microwave field and theoptical mode, resulting in high modulation efficiency. In FIG. 1C, thehandle for the ion-sliced lithium niobate is a silicon substrate,instead of lithium niobate. The optical mode resides primarily in thelithium niobate rib waveguide, but the microwave mode extends into thesilicon substrate. Tuning of the layer thicknesses in the ion-slicedlithium niobate, silicon dioxide, and silicon substrate allows themicrowave and optical group velocities to be designed with moreflexibility. Consequently, velocity matching can be engineered moreeasily without reducing the electro-optical overlap. Optical waveguidescan be etched with smooth sidewalls, allowing for optical propagationlosses of 0.2 dB/cm. Tapered lensed fibers allow for a coupling loss ofabout 5 dB/facet.

The state-of-the-art modulator, shown in FIG. 1C, does not allow formicrowave and optical waveguide crossings. The waveguide crossings arerequired for packaging the modulator with high-frequency co-axialconnectors and with standard telecommunications wavelength opticalfiber. One solution is to introduce a buffer layer between the opticalwaveguide and the metal electrodes, as shown in the approach of FIG. 1B.The buffer layer allows for optical waveguides to pass underneath themetal electrodes. Unfortunately, this approach comes at the expense ofincreasing the product of the half-wave voltage and the modulationlength, V_(π)L, because the overlap between the optical mode and themicrowave mode is reduced. Another solution is to utilize microwave viasand a second metallization layer. Ultimately, however, as modulationfrequency is pushed beyond 100 GHz, the use of microwave vias will be abandwidth limiting approach due to the inductance introduced.

Furthermore, the state-of-the-art modulator, shown in FIG. 1C, does notproduce efficient coupling to standard optical fiber that is requiredfor the insertion of the modulator as a component in a larger system.Non-standard specialty fiber in the form of lensed fibers are used tobutt-couple light into and out of the chip. The coupling loss issubstantial, about 5 dB per facet, which can be prohibitively large whenincorporated into fielded systems.

It is with respect to these and other considerations that the variousaspects and embodiments of the present disclosure are presented.

SUMMARY

A fiber-to-fiber system for multi-layer ferroelectric on insulatorwaveguide devices is described. The system comprises a fiber-to-chipcoupler that couples light from a standard optical fiber to multi-layerferroelectric on insulator waveguides. The multi-layer ferroelectric oninsulator waveguides are integrated with electrodes to implement anoptical device, an electro-optical device, or a non-linear opticaldevice, such as an electro-optical modulator, with microwave and opticalwaveguide crossings compatible with packaging. A second fiber-to-chipcoupler outputs the light from the multi-layer ferroelectric oninsulator device to a standard optical fiber.

In an implementation, a system comprises: a first fiber-to-chip couplerconfigured to receive light from a first optical fiber; a plurality ofmulti-layer ferroelectric on insulator optical waveguides; and a secondfiber-to-chip coupler configured to output light to a second opticalfiber, wherein the multi-layer ferroelectric on insulator waveguides arecoupled between the first fiber-to-chip coupler and the secondfiber-to-chip coupler.

In an implementation, a system comprises: a first fiber-to-chip couplerconfigured to receive light from a first optical fiber; a plurality ofmulti-layer ferroelectric on insulator optical waveguides andnon-ferroelectric on insulator waveguides; and a second fiber-to-chipcoupler configured to output light to a second optical fiber, whereinthe multi-layer ferroelectric on insulator optical waveguides and thenon-ferroelectric on insulator optical waveguides are coupled betweenthe first fiber-to-chip coupler and the second fiber-to-chip coupler.

In an implementation, a method comprises: forming a first insulatinglayer on a handle wafer; implanting a donor wafer with ions to form adamage layer beneath the surface; bonding the donor wafer and the handlewafer; annealing the bonded wafers to exfoliate a first film along theion implant damage layer; forming a second insulating layer on the firstfilm; implanting a donor wafer with ions to form a damage layer beneaththe surface; bonding the donor wafer to the second insulating layer onthe first film; and annealing the bonded wafers to exfoliate a secondfilm along the ion implant damage layer.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofillustrative embodiments, is better understood when read in conjunctionwith the appended drawings. For the purpose of illustrating theembodiments, there is shown in the drawings example constructions of theembodiments; however, the embodiments are not limited to the specificmethods and instrumentalities disclosed. In the drawings:

FIGS. 1A, 1B, and 1C are illustrations of various prior artelectro-optical modulator cross-sections including conventionalTi-diffused waveguides in bulk LiNbO₃; strip loaded ion-sliced LiNbO₃waveguides; and etched ion-sliced LiNbO₃ waveguides on Si, respectively;

FIG. 2 is an illustration of an implementation of a fiber-to-fibersystem for multi-layer ferroelectric on insulator waveguide devices;

FIG. 3 is an illustration of an implementation of an electro-opticalmodulator cross-section with multi-layer ion-sliced LiNbO₃ waveguides;

FIG. 4A shows a top-down view of an implementation of an electro-opticalmodulator with multi-layer optical waveguides;

FIG. 4B shows optical modeling of vertical coupling between waveguides;

FIGS. 5A-5F shows modeling results of an implementation of anelectro-optical modulator: electro-optical (EO) response (FIG. 5A), RFLoss (FIG. 5B), RF effective index (FIG. 5C), real part of Z_(o) (FIG.5D), imaginary part of Z_(o) (FIG. 5E), and RF group index (FIG. 5F);

FIG. 6 shows a diagram of an implementation of a fabrication process forlithium niobate on insulator on a silicon handle wafer;

FIG. 7 is an illustration of computed total strain energy U, principalnormal stress n, and principal in-plane stress σ₃ of the LN wafer versusthickness at 190° C.;

FIG. 8 shows surface profilometry measurements of the LN donor and thedeformed silicon handle wafer;

FIGS. 9A and 9B show atomic force microscopy scans of surface and edge,respectively, of ion-sliced LN bonded to oxidized silicon wafer;

FIG. 10 shows a cross-sectional scanning electron micrograph of LNOI onSi prior to chemical mechanical polishing;

FIG. 11A shows an illustration of an etched lithium niobate rib;

FIG. 11B shows expected optical propagation loss for etched LNOI ribwaveguides;

FIG. 12 shows an implementation of a process flow to realize multi-layerion-sliced lithium niobate waveguides;

FIG. 13 shows a fundamental mode in an LNOI waveguide obtained via asemi-vector beam propagation method at 1550 nm wavelength;

FIG. 14 shows an example fiber-to-chip coupler;

FIG. 15 shows an implementation of a fiber-to-chip coupler fabricationprocess;

FIG. 16 shows physical dimensions for a representative design of afiber-to-chip coupler;

FIG. 17 shows three-dimensional modeling of a fiber-to-chip couplingloss versus cantilever length with refractive index as a parameter;

FIG. 18 shows three-dimensional modeling of coupling loss versuscantilever length with rib-to-slab lithographic alignment error as aparameter;

FIG. 19 shows three-dimensional modeling of coupling loss versus fiberto coupling facet misalignment;

FIG. 20 shows three-dimensional modeling of coupler loss versuswavelength;

FIGS. 21A, 21B, and 21C, respectively show one, two, and three layers offerroelectric on insulator;

FIGS. 22A, 22B, and 22C, respectively show an example periodically poledwavelength conversion device and the optical modes at 1550 nm and 775 nmwavelengths;

FIG. 23 is an illustration of an implementation of an electro-opticalmodulator cross-section with silicon nitride;

FIG. 24 is an illustration of an implementation of an electro-opticalmodulator cross-section with electrical contacts to both lithium niobatelayers; and

FIG. 25 is an illustration of an implementation of a fiber-to-fibersystem for multi-layer ferroelectric on insulator waveguide deviceswhere a single physical optical fiber functions as the input and theoutput fiber.

DETAILED DESCRIPTION

This description provides examples not intended to limit the scope ofthe appended claims. The figures generally indicate the features of theexamples, where it is understood and appreciated that like referencenumerals are used to refer to like elements. Reference in thespecification to “one embodiment” or “an embodiment” or “an exampleembodiment” means that a particular feature, structure, orcharacteristic described is included in at least one embodimentdescribed herein and does not imply that the feature, structure, orcharacteristic is present in all embodiments described herein.

As described further herein, an implementation comprises (1) afiber-to-chip coupler that allows light to be coupled from a standardsingle mode optical fiber into a lithium niobate on insulator (LNOI)waveguide, (2) multi-layer coupled waveguides that allow light to becoupled vertically from a lithium niobate waveguide on one level to alithium niobate waveguide on another level, (3) electro-optical andnonlinear optical devices in multi-layer LNOI waveguides, and (4) asecond fiber-to-chip coupler that allows light to be coupled from a LNOIwaveguide into a standard single mode optical fiber.

Waveguides in a thin layer of a ferroelectric material such as lithiumniobate enable efficient wavelength conversion and electro-opticaldevices due to high spatial confinement of light. Exploiting the highspatial confinement introduces major difficulties, however, whenattempting to cross optical waveguides with metal electrodes, and whenattempting to couple light from the waveguides to standard opticalfiber. Waveguide crossings and coupling to standard optical fiber allowfor packaging that is required for field use and/or for using thewaveguide device as a component in a larger system. The implementationsdescribed herein provide a fiber-to-fiber solution that allows theadvantages of LNOI integrated photonic chips to be exploited in realworld fielded systems.

Implementations described herein provide the ability to produce fullypackaged devices that can be inserted into systems via standard opticalfiber used in current computing, communications, and sensing systems.Industry applications include classical and quantum computing,communications, and sensing. For example, electro-optical modulatorsimplemented with aspects described herein meet the need for developingradio frequency (RF) links that operate at frequencies over 100 GHz forultra-wideband high-dynamic range receivers. In another example,wavelength converters, electro-optic devices, and nonlinear opticaldevices implemented with aspects described herein meet the need fordeveloping components and transducers that can interconnect quantum bits(qubits) based on, for example, superconducting circuits or trappedions, to standard optical fiber for creating hybrid quantum networks,distributed quantum computers, and technology for long distancecommunication over the future quantum internet.

A fiber-to-fiber system for multi-layer ferroelectric on insulatorwaveguide devices is described. FIG. 2 is an illustration of animplementation of a fiber-to-fiber system 200 for multi-layerferroelectric on insulator waveguide devices. The system 200 comprises afiber-to-chip coupler 220 that couples light from a standard opticalfiber 210 to multi-layer ferroelectric on insulator waveguides 230. Themulti-layer ferroelectric on insulator waveguides 230 are integratedwith electrodes to implement an optical device, such as anelectro-optical modulator, with microwave and optical waveguidecrossings compatible with packaging. A second fiber-to-chip coupler 240outputs the light from the multi-layer ferroelectric on insulator device230 to a standard optical fiber 250.

The multi-layer approach is illustrated in FIG. 3 for the case where theferroelectric material is lithium niobate. FIG. 3 is an illustration ofan implementation of an electro-optical modulator 300 cross-section withmulti-layer ion-sliced LiNbO₃ waveguides. The multi-layer approachintroduces a second optical waveguide layer to allow for opticalwaveguide routing underneath the metal electrodes. Optical light in thetop waveguide layer 310 is coupled vertically downward to the bottomwaveguide layer 320 via vertical directional coupling. Similarly,optical light in the bottom waveguide layer 320 is coupled verticallyupward to the top waveguide layer 310 via vertical directional coupling.Light in one waveguide layer can couple to the other waveguide layer ina short longitudinal distance when the waveguides are in close verticalproximity.

FIG. 4A shows a top-down view of an implementation of an electro-opticalmodulator with multi-layer optical waveguides, and FIG. 4B shows opticalmodeling of vertical coupling between waveguides. For specificity, themulti-layer waveguide cross-section is that of FIG. 3, the opticalwavelength is 1550 nm, the rib waveguide width is 1350 nm, the ribwaveguide height is 100 nm, the rib waveguide slab height is 250 nm, theburied oxide is 4000 nm, and the vertical center-to-center spacingbetween waveguides is 1000 nm.

In FIG. 4A, a top-down schematic of the electro-optical modulator 400 isshown with optical waveguides on the metal layer 410 for low V_(π)L andoptical waveguides below the metal layer 420 for routing underneath themetal electrodes 430. Optical modeling, shown in FIG. 4B, shows thatvertical coupling occurs over a length of 43 μm for a vertical gap of 1μm, allowing for clearance from metal electrodes. The opticalpropagation loss from the metal (gold) for 1550 nm light in the bottomwaveguide is only 0.95 dB/cm.

The top-down view of the electro-optical modulator 400 in FIG. 4Aillustrates that the systems and methods described herein allows forpackaging of the chip with west-to-east input and output optical fibers,and south input/output microwave connectors electrically connected tothe co-planar waveguide electrodes. North/south sides of the packagingcan be used for high-frequency and DC electrical connections to thechip.

FIGS. 5A-5F shows modeling results of an implementation of anelectro-optical modulator: electro-optical (EO) response (FIG. 5A), RFLoss (FIG. 5B), RF effective index (FIG. 5C), real part of Z_(o) (FIG.5D), imaginary part of Z_(o) (FIG. 5E), and RF group index (FIG. 5F).For specificity, the optical wavelength is 1550 nm, the rib waveguidewidth is 1350 nm, the rib waveguide height is 100 nm, the rib waveguideslab height is 250 nm, the buried oxide is 4000 nm, the metal height is1100 nm, the center electrode width is 40 μm, the co-planar waveguidelateral gap spacing is 6.35 μm, and the finite ground width of theco-planar waveguide is 120 μm.

The finite element method was utilized using COMSOL™ to establishexpected modulator performance, shown in FIGS. 5A-5F. A V_(π)L of 4.8 Vcm and 110 GHz 3 dB bandwidth are expected in a modulator length of 5mm. At 110 GHz, the RF loss is 7 dB/cm, the RF CPW mode effective indexis 2.12, the characteristic impedance (Z_(O)) is 39.3Ω, and the RF groupindex is 2.12. At 1550 nm wavelength, the optical group index is 2.18.The modulator performance meets the need for developing radio frequency(RF) links that operate at frequencies up to 110 GHz for ultra-widebandhigh-dynamic range receivers.

FIG. 6 shows a diagram of an implementation of a fabrication process 600for lithium niobate on insulator on a silicon handle wafer. Ion-slicingis used to realize LNOI. For specificity, the ion-sliced LN thickness is800 nm. As illustrated in FIG. 6, the process starts with a lithiumniobate (LN) donor wafer 610 and a silicon handle wafer 615. The siliconwafer is oxidized to produce a silicon dioxide (SiO₂) layer 620. Thedonor wafer is implanted with He⁺ ions 625 to form a damage layer 630below the +x surface. The depth of the damage layer 630 depends on theimplantation energy and is calculated using Monte Carlo simulations.After implantation, both wafers are bonded 640 at room temperature. Thebonded wafers are then annealed 645 to exfoliate 650 a film 655 alongthe ion implant damage layer. The top surface of the lithium niobatethin film 655 is polished to reduce surface roughness using, forexample, chemical mechanical polishing techniques. In an implementation,magnesium oxide (MgO) doped LN thin film (MgO:LNOI) on LN handle over a3″ wafer may be fabricated. Furthermore, the donor and/or handle waferscould be implemented as dies instead of as full wafers.

Bonding to a silicon substrate is advantageous for electronic-photonicintegration but is challenging because of debonding and cracking due tothermal expansion coefficient (TEC) mismatch between Si and LN.Fabrication of ion sliced LNOI on a Si handle wafer is achieved here byselecting optimized wafer thicknesses informed by structural modelingand accommodating for dissimilar wafer bows using a bonding apparatus.

FIG. 7 is an illustration 700 of computed total strain energy U,principal normal stress σ₁, and principal in-plane stress σ₃ of the LNwafer versus thickness at 190° C. FIG. 7 shows an LN-oxide-Si structuralfinite element analysis of strain energy and stress, due to thermalexpansion coefficient (TEC) mismatch at elevated temperatures, in theabsence of He⁺ implantation. The LN donor wafer and Si substratediameters are 3 and 4 inches, respectively, and the oxide is 4 μm thick.High strain energy and stress can result in debonding and cracking,respectively. The analysis in FIG. 7 shows that thinner LN wafers areexpected to be more robust at elevated temperatures required forexfoliation.

For ion-slicing, an x-cut LN donor substrate of 0.25 mm thickness isimplanted with He ions at 225 keV with a fluence of 3.5×10¹⁶ cm⁻². A dieof implanted LN and a thermally oxidized Si handle wafer are cleanedusing wet chemistry and directly bonded at room temperature. Thinimplanted LN exhibits large wafer bow, which is not ideal for bonding,relative to the bow of thick implanted LN. Bonding of the 0.25 mm thickLN die is facilitated, however, though a jig that deforms the Si handleto match the bow of the LN sample, verified by surface profilometry 800shown in FIG. 8. FIG. 8 shows surface profilometry measurements of theLN donor and the deformed silicon handle wafer. The bonded pair isannealed at 188° C. to exfoliate and transfer the LN thin film onto theoxidized Si wafer. Successful cm-scale film transfer is achieved. FIGS.9A and 9B show atomic force microscopy scans 900, 950 of surface andedge, respectively, of ion-sliced LN bonded to oxidized silicon wafer.FIGS. 9A and 9B show a root mean square surface roughness of 5.6 nm anda transferred LN film thickness of 817 nm, measured by atomic forcemicroscopy. Film thickness measurements agree with Transport of Ions inMatter (TRIM) calculations of 807 nm. A scanning electron micrograph1000 of fabricated LNOI is shown in FIG. 10. FIG. 10 shows across-sectional scanning electron micrograph of LNOI on Si prior tochemical mechanical polishing (CMP). The CMP process planarizes the LNto sub-nanometer root mean square surface roughness, which removes theeffect of helium blistering that is visible on the surface of the LN inFIG. 10.

Etching of the thin film of lithium niobate is used to form, forexample, rib or strip waveguides. Lithium niobate is etched via plasmaetching using, for example, Ar and CHF₃ chemistry. An illustration 1100of an etched lithium niobate rib is shown in FIG. 11A. FIG. 11B showsexpected optical propagation loss for etched LNOI rib waveguides withrib height (Ti) as parameter. T₂ is the unetched lithium niobate thinfilm thickness. Rib width is 1350 nm. The wavelength is 1550 nm. Asshown in FIG. 11B, a sidewall surface scattering loss of less than 0.1dB/cm is estimated. Optimization of the surface roughness is likelybound by the surface roughness of etch masks. With processingoptimization, it is expected to be able to achieve an optical waveguidepropagation loss of 0.1 dB/cm or less.

Patterned metal electrodes may be used to form an electro-opticalmodulator. Gold is deposited on the etched lithium niobate ribwaveguides and then patterned to form metal electrodes. Microwavemodeling is conducted to design the electrodes. Optical modeling isconducted to design the waveguides. Both models are integrated to designelectro-optical overlap. Based on modeling, it is expected to be able toachieve V_(π)L<5 V cm, 100 GHz EO bandwidth for 5 mm modulation length,and 35 GHz EO bandwidth for 10 mm modulation length in fabricateddevices.

Fabrication of multi-layer ion-sliced lithium niobate waveguides isdescribed. FIG. 12 shows an implementation of a process flow 1200 torealize multi-layer ion-sliced lithium niobate waveguides. Start at 1210by depositing an insulating film such as plasma enhanced chemical vapordeposition (PECVD) silicon dioxide on top of etched lithium niobate ribwaveguides. The surface is then planarized by chemical mechanicalpolishing. Next, at 1220, a second donor wafer is ion-implanted andbonded to the planarized silicon dioxide. After annealing andexfoliation, the second ion-sliced lithium niobate thin film ispatterned and etched at 1230 to form the top layer of rib waveguides. At1240, a metal such as gold is deposited and electrodes are patterned.

Optical lithography allows for a layer-to-layer alignment on the orderof 250 nm and electron beam lithography allows for layer-to-layeralignment on the order of 50 nm. Modeling shows negligible insertionloss for no lateral offset error and 0.31 dB excess insertion loss for alateral offset error of 250 nm. For specificity, the optical wavelengthis 1550 nm, the rib waveguide width is 1350 nm, the rib waveguide heightis 100 nm, the rib waveguide slab height is 250 nm, the buried oxide is4000 nm, and the vertical center-to-center spacing between waveguides is1000 nm. Bonding processes and annealing temperatures are optimized suchthat the first layer of lithium niobate waveguides withstands thestresses involved during fabrication of the second layer of waveguides,and electrodes, on top. Characterization of the efficiency of verticaldirectional coupling over a range of devices that are fabricated withinthe 250 nm alignment tolerance is accomplished using optical throughputmeasurements. A vertical directional coupler insertion loss <0.5 dB infabricated devices is expected.

A fiber-to-chip coupler is described. The high confinement enabled bythin films of lithium niobate produces high performance electro-opticaland nonlinear optical waveguide devices but also introduces majorchallenges when attempting to efficiently couple light between lithiumniobate waveguides and optical fibers. Large coupling losses are theresult of effective index mismatch, mode size mismatch, and mode fielddistribution mismatch between a standard optical fiber and a waveguidemode in lithium niobate on insulator. While the mode field diameter ofoptical fiber at 1550 nm wavelength is on the order of 10 μm, the modefield diameter of a waveguide mode in LNOI is on the order of onemicrometer. FIG. 13 shows a fundamental mode in an LNOI waveguideobtained via a semi-vector beam propagation method. For specificity, theoptical wavelength is 1550 nm, the rib waveguide width is 1350 nm, therib waveguide height is 100 nm, the rib waveguide slab height is 250 nm,and the buried oxide is 4000 nm.

Current methods designed to achieve efficient fiber-to-chip couplinggenerally involve edge coupling using lensed fibers or surface couplingusing grating couplers with standard fibers. Lensed fibers improve theedge coupling but are non-standard fibers and require an air gap betweenthe lensed fiber and the chip. Furthermore, the edge of the chip needsto be cleaved and polished. Alternatively, grating couplers used withstandard fibers enable light coupling via the surface of the chipwithout the need for cleaving and polishing. They require, however, atradeoff between bandwidth and efficiency.

Design and fabrication of a fiber-to-chip coupler is described. Here,the concept of cantilever couplers is adapted to ferroelectric oninsulator material systems with some changes. First, the optical fiberis no longer a tapered optical fiber and is instead a standard (SMF-28)cleaved optical fiber. The change removes the requirement for specialtyoptical fiber. Second, a fiber alignment slot is integrated into thecantilever design to allow for in-line alignment and for the permanentpackaging of the optical fiber to the cantilever via epoxy, UV curedoptical adhesive, or index matching adhesive. Third, the coupler designis changed to a rib waveguide design, instead of a strip waveguidedesign.

An example fiber-to-chip coupler 1400 is shown in FIG. 14. The couplerallows the use of standard SMF-28 fibers, or standard polarizationmaintaining fibers, to efficiently couple light into ferroelectric oninsulator waveguides. Starting at the far left of the diagram, an SMF-28single mode fiber 1410 is edge coupled to the ferroelectric on insulatorchip 1420. A groove is patterned in the handle wafer to providealignment and fixation of the optical fiber to the fiber-to-chip couplercantilever. The fiber-to-chip coupler 1400 is a suspended inverse-widthtaper ferroelectric on insulator waveguide encapsulated in a silicondioxide cantilever.

FIG. 15 shows an implementation of a fiber-to-chip coupler fabricationprocess flow 1500 in the context of lithium niobate on insulatorwaveguides. The first step is to begin with a lithium niobate (LN) thinfilm bonded to an oxidized silicon wafer, illustrated in (a). The secondstep, shown in (b), is to fully etch the LN thin film to form an inversewidth slab taper via lithography and plasma etching. The slab taperwidth increases, following a curve designed to optimize couplingefficiency. Near the edge of the chip, a notch in the LN is etched toassist in fiber alignment. The third step, shown in (c), is to pattern aLN tapered rib on top of the LN slab. The rib is aligned to the taperedslab. The rib taper width increases, following a curve designed tooptimize coupling efficiency. Together, the tapered slab and the taperedrib form a tapered LN rib waveguide. The fourth step, shown in (d), isto deposit silicon oxide via plasma enhanced chemical vapor deposition(PECVD). The PECVD silicon oxide covers the entire wafer and serves as atop waveguide cladding. The fifth step, shown in (e), is to etch siliconoxide pits to form an oxide cantilever that encloses the tapered LN ribwaveguide. In this step, PECVD silicon oxide and thermal oxide, from theoxidized silicon wafer, are etched using plasma etching to define thecantilever, support struts for the cantilever, and the optical fibernotch. The cantilever terminates in the optical fiber notch. A shortoffset distance is present from the end of the LN ridge to the end ofthe cantilever. The sixth step, shown in (0 is to isotropically etch thesilicon substrate to undercut the silicon oxide. The undercut forms afree-standing oxide cantilever with the tapered silicon rib embedded.The mechanical struts on the side of the cantilever are optional. Theseventh step, shown in (g), is a further deep etch of the silicon toallow for edge coupling and alignment using standard SMF-28 fiber at theend notch, as shown in (h).

FIG. 16 shows physical dimensions for a representative design of afiber-to-chip coupler 1600. The LN thin film thickness is 600 nm, thethickness of the thermal silicon dioxide layer is 4 μm, and thethickness of the silicon handle wafer is 500 μm. The length of the slabtaper is 156 μm. The smallest width of the slab taper is 50 nm. The slabtaper width increases, following a third order Bezier curve, to a widthof 6 μm. The width of the notch is 145 μm. The LN tapered rib is 200 nmtall on top of the LN slab. The smallest dimension of the rib taper is50 nm. The rib taper width increases, following a third order Beziercurve, to a width of 3.1 μm. The deposited PECVD silicon oxide is 4 μm.The offset distance from the end of the LN ridge to the end of thecantilever is 4 μm. The total cantilever length is 160 μm. Thecantilever width is 8 μm. The pits are 20 μm wide laterally in thedirection perpendicular to the longitudinal axis of the rib. The depthof the notch is 68.5 μm, measured from the Si/SiO₂ interface.

Modeling and simulation of a representative design of a fiber-to-chipcoupler is described. To couple via standard SMF-28 fiber, thecantilever pits are filled with commercially available index matchingfluid with a refractive index of 1.425 at 1550 nm wavelength. Therefractive index contrast to SiO₂ at 1550 nm wavelength is 0.02 given arefractive index of 1.445 for SiO₂ at 1550 nm wavelength. Numericalmodeling using three-dimensional finite difference time domain (FDTD)simulations are shown in FIG. 17. The parameter in the legend is theindex of refraction of the index matching fluid. For a cantilever lengthof 160 μm, the coupling loss is only 0.4 dB at 1550 nm wavelength,representing the state-of-the-art.

The alignment between the rib and the slab during fabrication isidentified as a critical parameter. FIG. 18 shows three-dimensionalmodeling of coupling loss versus cantilever length with rib-to-slablithographic alignment error as a parameter at 1550 nm wavelength. At acoupler length of 160 μm the coupling loss changes by less than 0.25 dBfor rib-to-slab alignment error of up to 500 nm which is well withinalignment capability of lithography.

The alignment between the SMF-28 fiber and the cantilever is alsoidentified as a critical parameter. FIG. 19 shows three-dimensionalmodeling of coupling loss versus fiber misalignment with cantileverfacet. As shown in FIG. 19, at a coupler length of 160 μm at 1550 nmwavelength, the excess coupling loss is 0.52 dB for 1.5 μm misalignmentin either the vertical or lateral directions.

FIG. 20 shows three-dimensional modeling of coupler loss versuswavelength. The coupling loss varies by 0.4 dB from 1.35 μm to 1.75 μm.The fiber-to-chip coupler operates over a wide optical bandwidth.

With respect to fully packaged devices, packaged optical devices,electro-optical devices, and nonlinear optical devices, such aselectro-optical modulators, are enclosed in aluminum casings with fiberpigtails and microwave/millimeter-wave co-axial connectors. Input andoutput 50/50 directional couplers are on the lower rib waveguides andstraight parallel waveguides are on the upper rib waveguides. Designoptimization is accomplished via electromagnetic modeling that isinformed by experimental characterization of fabricated passive andactive devices. Expected are V_(π)L<5 V cm, 100 GHz EO bandwidth for 5mm modulation length, 35 GHz EO bandwidth for 10 mm modulation length,and fiber-to-fiber loss of 7.7 dB. The fiber-to-fiber loss includes loss(with margin) from fiber-to-chip coupling (1 dB×2=2 dB), on-chip topwaveguide propagation loss (0.2 dB/cm×5 mm=0.1 dB), on-chip bottomwaveguide propagation loss (0.2 dB/cm×3 mm=0.06 dB), vertical couplerinsertion loss (0.5 dB×4=2 dB), 50/50 splitter insertion loss (1 dB×2=2dB), and fiber pigtail connector loss (0.75 dB×2=1.5 dB).

Generalizing, in the context of multi-layer ferroelectric on insulatorwaveguides, the number of optical waveguide layers can be one, two, ormore. FIGS. 21A, 21B, and 21C, respectively show one, two, and threelayers 2110 of ferroelectric on insulator. The presence of metalelectrodes is optional. Also, the waveguide device in lithium niobate oninsulator is not limited to electro-optical modulators. The waveguidedevice can also be a wavelength conversion device such as a secondharmonic generator, a tunable lasing device such as an opticalparametric oscillator, and also nonlinear optical devices based on sumfrequency generation, difference frequency generation, opticalrectification, and spontaneous parametric down conversion.

Generalizing, the electrodes can function as waveguides forhigh-frequency electrical signals, as in the case of co-planarwaveguides (CPW) for the electro-optical modulator shown in FIG. 4A, aselectrical contacts for introducing a DC bias for the modulator, or aspoling electrodes for electrically tunable quasi-phase matched nonlinearoptical devices such as wavelength converters and optical parametricoscillators. On-chip poling electrodes used to electrically control thenonlinear optical output by dynamically poling and un-poling benefitfrom the metal and optical waveguide crossings enabled by themulti-layer waveguides by providing flexibility in chip layout andaccess to chip edges for packaging.

Generalizing, the optical rib waveguide can be formed by depositing andpatterning a thin film of a dielectric material such as silicon nitride.Such strip loading is an alternative to etching the rib in theferroelectric.

An example wavelength conversion device in the form of a second harmonic(SH) generator is shown in FIG. 22A. FIGS. 22A, 22B, and 22C,respectively show an example periodically poled wavelength conversiondevice and the optical modes at 1550 nm and 775 nm wavelengths. On-chipelectrodes are used to periodically pole the lithium niobate forquasi-phase matching, resulting in alternating direction of thespontaneous polarization in the lithium niobate. The rib waveguide isformed by strip loading with silicon nitride. Experimental results showthat the second harmonic power scales quadratically with the fundamentalpower, as expected from theory. The second harmonic generationefficiency is 600% W⁻¹ cm⁻². The on-chip electrodes allow for poling andfor the electrical control of optical second harmonic output. Thewavelength conversion device meets the need for developing efficientnonlinear optical waveguide components for classical and quantumcommunications applications. In the context of quantum informationscience and technology, components and transducers that can interconnectquantum bits (qubits) based on, for example, superconducting circuits ortrapped ions, to standard optical fiber are needed to create hybridquantum networks, distributed quantum computers, and technology for longdistance communication over the future quantum internet.

Generalizing, in the context of multi-layer ferroelectric on insulatorwaveguide fabrication, the donor wafer can be undoped LN or doped LNwith magnesium oxide (MgO) to reduce photorefractive damage. The donorwafer can also be another ferroelectric material such as potassiumtitanyl phosphate KTiOPO₄ (KTP). The handle wafer 615 is silicon in FIG.6 but can also be a wafer of another material such as lithium niobate,KTP, quartz, sapphire, or silicon carbide. In the case of a siliconhandle wafer, the silicon wafer is oxidized to produce a silicon dioxide(SiO₂) layer. For handle wafers composed of other materials such aslithium niobate, KTP, quartz, sapphire, or silicon carbide, aninsulating layer, such as a silicon oxide layer, is deposited on thehandle wafer. The insulating layer is optional for handle wafers, suchas quartz and sapphire, that possess indices of refraction less than theindex of refraction of lithium niobate. Furthermore, the LN thin filmthickness can range from a monolayer to 10 μm, the silicon dioxide layercan range from a monolayer to 10 μm, and the silicon handle thicknesscan range from 50 μm to 1000 μm or more. Post fabrication, the siliconsubstrate handle can be removed by, for example, wet etching, forflexible photonic device applications.

Generalizing, in the context of multi-layer ferroelectric on insulatorwaveguide design and fabrication, one layer of waveguides can becomposed of a material of one type of crystal cut that is different thanthe crystal cut of the other waveguide layer. For example, the topwaveguide layer can be x-cut lithium niobate and the bottom waveguidelayer can be z-cut lithium niobate, or vice-versa.

Generalizing, in the context of multi-layer ferroelectric on insulatorwaveguide design and fabrication, one layer of waveguides can becomposed of a material other than a ferroelectric. For example, for anelectro-optical modulator, the bottom layer of waveguides can becomposed of silicon nitride or silicon. FIG. 23 is an illustration of animplementation of an electro-optical modulator 2300 cross-section withsilicon nitride. FIG. 23 shows a top waveguide layer 2310 is lithiumniobate and the bottom waveguide layer 2320 is silicon nitride. Thebottom waveguide layer 2320 is passive. Vertical coupling occurs mostefficiently when the mode propagation constants of two waveguides arethe same. Silicon nitride can exhibit an index of refraction similar tolithium niobate. Both index of refraction and waveguide geometrydetermine the mode propagation constant. When silicon nitride or siliconis used as the bottom waveguide layer 2320, the fiber-to-chip coupler isbased on silicon nitride or silicon waveguides.

Generalizing, in the context of vertical directional coupling, thedesign of two waveguides involved in vertical directional coupling canexploit adiabatic tapers or two different lateral and vertical waveguidecross-sectional dimensions to reduce the fabrication tolerance requiredfor efficient vertical directional coupling. A resonator, such as a ringor disk resonator, can be sandwiched between the two vertical waveguidesin order to introduce frequency selectivity to the vertical coupling.Waveguide gratings can also be utilized to achieve vertical coupling.

Generalizing, in the context of vertical directional coupling, whensilicon nitride is used for a waveguide layer, forming silicon nitriderib or strip waveguides, the optical functionality can be passive, orcan be based on properties, such as third order nonlinearities, inherentto silicon nitride. When lithium niobate is used for a waveguide layer,forming lithium niobate strip or rib waveguides, the opticalnonlinearity can be exploited for voltage control of optical phase, suchas in the case of the electro-optic effect, or for general nonlinearoptical effects, such as in the case of wavelength conversion, and wavemixing. Additional electrodes can be used to access the electro-opticeffect or optical nonlinearity in the bottom waveguide 2420, illustratedin FIG. 24. FIG. 24 is an illustration of an implementation of anelectro-optical modulator 2400 cross-section with electrical contacts toboth lithium niobate layers. In FIG. 24, the top waveguide layer 2410 islithium niobate and the bottom waveguide layer 2420 is lithium niobate.The layers 2410, 2420 are optical active via electro-optic effect andnonlinear optical effects. Additional electrodes can be used to controlthe optical phase in the bottom waveguide.

Generalizing, in the context of fiber-to-chip couplers, an example ofstandard optical fiber is SMF-28, used for optical telecommunications inthe optical C-band between 1530 nm and 1565 nm wavelength. Anotherexample is polarization maintaining fiber typically packaged withelectro-optical modulators. A third example is optical fiber designedfor operation in the visible wavelength regime. Physical dimensionsshown in FIG. 16 are for operation in the optical C-band. Parameters maybe optimized for operation at other wavelength bands, such aswavelengths near 1310 nm, and wavelengths in the range of visible lightless than 1000 nm. For example, in a wavelength conversion device, onefiber-to-chip coupler can be designed for operation at 1550 nm and thesecond fiber-to-chip coupler can be designed for operation at 775 nm.Furthermore, alternatively, the rib waveguide in the coupler can beformed by depositing and patterning a thin film of a dielectric materialsuch as silicon nitride. Such strip loading is an alternative to etchingthe rib in the ferroelectric.

Generalizing, in the context of fiber-to-fiber platform for multi-layerferroelectric on insulator waveguide devices, the input fiber can alsoserve as the output fiber. In this case, there is only one fiber-to-chipcoupler. The multi-layer ferroelectric on insulator device produces aback reflection so that the output fiber is the same as the input fiber,illustrated in FIG. 25. FIG. 25 is an illustration of an implementationof a fiber-to-fiber system 2500 for multi-layer ferroelectric oninsulator 2530 waveguide devices where a single physical optical fiber2510 functions as the input and the output fiber. FIG. 25 shows a singlefiber-to-chip coupler 2520 configuration can be used where one opticalfiber 2510 hosts the input and output light. In another configuration,the multi-layer ferroelectric on insulator device produces no backreflection so the output light propagates into free-space.

In an implementation, a system comprises: a first fiber-to-chip couplerconfigured to receive light from a first optical fiber; a plurality ofmulti-layer ferroelectric on insulator optical waveguides; and a secondfiber-to-chip coupler configured to output light to a second opticalfiber, wherein the multi-layer ferroelectric on insulator waveguides arecoupled between the first fiber-to-chip coupler and the secondfiber-to-chip coupler.

Implementations may include some or all of the following features. Themulti-layer ferroelectric on insulator waveguides comprise at least oneferroelectric on insulator waveguide, such as a lithium niobate oninsulator (LNOI) waveguide. The multi-layer ferroelectric on insulatorwaveguides comprise multi-layer coupled waveguides that allow light tobe coupled vertically from a ferroelectric waveguide on one level to aferroelectric waveguide on another level. The first fiber-to-chipcoupler is configured to allow light to be coupled from a single modeoptical fiber into a ferroelectric on insulator waveguide, and thesecond fiber-to-chip coupler is configured to allow light to be coupledfrom a ferroelectric on insulator waveguide into a second single modeoptical fiber. The multi-layer ferroelectric on insulator waveguides areintegrated with electrodes to implement an optical device, anelectro-optical device, or a non-linear optical device, such as anelectro-optical modulator, with microwave and optical waveguidecrossings, and electrical interconnects, compatible with packaging. Onephysical optical fiber functions as both the input and the outputoptical fibers, and one physical fiber-to-chip coupler functions as boththe input and output fiber-to-chip couplers.

In an implementation, a system comprises: a first fiber-to-chip couplerconfigured to receive light from a first optical fiber; a plurality ofmulti-layer ferroelectric on insulator optical waveguides andnon-ferroelectric on insulator waveguides; and a second fiber-to-chipcoupler configured to output light to a second optical fiber, whereinthe multi-layer ferroelectric on insulator optical waveguides and thenon-ferroelectric on insulator optical waveguides are coupled betweenthe first fiber-to-chip coupler and the second fiber-to-chip coupler.

Implementations may include some or all of the following features. Themulti-layer ferroelectric on insulator waveguides comprise multi-layercoupled waveguides that allow light to be coupled vertically from aferroelectric waveguide on one level to a non-ferroelectric waveguide onanother level. The first fiber-to-chip coupler is configured to allowlight to be coupled from a single mode optical fiber into aferroelectric on insulator waveguide, or into a non-ferroelectric oninsulator waveguide, and the second fiber-to-chip coupler is configuredto allow light to be coupled from a ferroelectric on insulator waveguideinto a single mode optical fiber, or coupled from a non-ferroelectric oninsulator waveguide into a single mode optical fiber. The multi-layerferroelectric and non-ferroelectric on insulator waveguides areintegrated with electrodes to implement an optical device, anelectro-optical device, or a non-linear optical device, such as anelectro-optical modulator, with microwave and optical waveguidecrossings, and electrical interconnects, compatible with packaging. Onephysical optical fiber functions as both the input and the outputoptical fibers, and one physical fiber-to-chip coupler functions as boththe input and output fiber-to-chip couplers.

In an implementation, a method comprises: forming a first insulatinglayer on a handle wafer; implanting a donor wafer with ions to form adamage layer beneath the surface; bonding the donor wafer and the handlewafer; annealing the bonded wafers to exfoliate a first film along theion implant damage layer; forming a second insulating layer on the firstfilm; implanting a donor wafer with ions to form a damage layer beneaththe surface; bonding the donor wafer to the second insulating layer onthe first film; and annealing the bonded wafers to exfoliate a secondfilm along the ion implant damage layer.

Implementations may include some or all of the following features. Thedonor wafer is a ferroelectric wafer, such as a lithium niobate wafer.The method further comprises patterning the thin film of theferroelectric to form optical waveguides. The method further comprisespattering electrodes and/or microwave waveguides in proximity to thefirst and/or second films. At least one of the handle wafer or the donorwafer are die. The bow of the handle is matched to the bow of the donor.A non-ferroelectric thin film material is formed on the first or secondinsulating layers by deposition, in lieu of exfoliation, resulting inone ferroelectric thin film and one non-ferroelectric thin film. Theferroelectric and non-ferroelectric thin films are patterned intooptical waveguides. The process is repeated to form multilayer opticalwaveguides where the number of waveguide layers is greater than two.

As used herein, the singular form “a,” “an,” and “the” include pluralreferences unless the context clearly dictates otherwise. As usedherein, the terms “can,” “may,” “optionally,” “can optionally,” and “mayoptionally” are used interchangeably and are meant to include cases inwhich the condition occurs as well as cases in which the condition doesnot occur.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Itwill be further understood that the endpoints of each of the ranges aresignificant both in relation to the other endpoint, and independently ofthe other endpoint. It is also understood that there are a number ofvalues disclosed herein, and that each value is also herein disclosed as“about” that particular value in addition to the value itself. Forexample, if the value “10” is disclosed, then “about 10” is alsodisclosed.

Although exemplary implementations may refer to utilizing aspects of thepresently disclosed subject matter in the context of one or morestand-alone computer systems, the subject matter is not so limited, butrather may be implemented in connection with any computing environment,such as a network or distributed computing environment. Still further,aspects of the presently disclosed subject matter may be implemented inor across a plurality of processing chips or devices, and storage maysimilarly be effected across a plurality of devices. Such devices mightinclude personal computers, network servers, and handheld devices, forexample.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed:
 1. A system comprising: a first fiber-to-chip couplerconfigured to receive light from a first optical fiber; a plurality ofmulti-layer ferroelectric on insulator optical waveguides; and a secondfiber-to-chip coupler configured to output light to a second opticalfiber, wherein the multi-layer ferroelectric on insulator waveguides arecoupled between the first fiber-to-chip coupler and the secondfiber-to-chip coupler.
 2. The system of claim 1, wherein the multi-layerferroelectric on insulator waveguides comprise multi-layer coupledwaveguides that allow light to be coupled vertically from aferroelectric waveguide on one level to a ferroelectric waveguide onanother level.
 3. The system of claim 1, wherein the first fiber-to-chipcoupler is configured to allow light to be coupled from a single modeoptical fiber into a ferroelectric on insulator waveguide, and thesecond fiber-to-chip coupler is configured to allow light to be coupledfrom a ferroelectric on insulator waveguide into a second single modeoptical fiber.
 4. The system of claim 1, wherein the multi-layerferroelectric on insulator waveguides are integrated with electrodes toimplement an optical device, an electro-optical device, or a non-linearoptical device, such as an electro-optical modulator, with microwave andoptical waveguide crossings, and electrical interconnects, compatiblewith packaging.
 5. The system of claim 1, wherein one physical opticalfiber functions as both the input and the output optical fibers, and onephysical fiber-to-chip coupler functions as both the input and outputfiber-to-chip couplers.
 6. A system comprising: a first fiber-to-chipcoupler configured to receive light from a first optical fiber; aplurality of multi-layer ferroelectric on insulator optical waveguidesand non-ferroelectric on insulator waveguides; and a secondfiber-to-chip coupler configured to output light to a second opticalfiber, wherein the multi-layer ferroelectric on insulator opticalwaveguides and the non-ferroelectric on insulator optical waveguides arecoupled between the first fiber-to-chip coupler and the secondfiber-to-chip coupler.
 7. The system of claim 6, wherein the multi-layerferroelectric on insulator waveguides comprise at least oneferroelectric on insulator waveguide, such as a lithium niobate oninsulator (LNOI) waveguide.
 8. The system of claim 6, wherein themulti-layer ferroelectric on insulator waveguides comprise multi-layercoupled waveguides that allow light to be coupled vertically from aferroelectric waveguide on one level to a non-ferroelectric waveguide onanother level.
 9. The system of claim 6, wherein the first fiber-to-chipcoupler is configured to allow light to be coupled from a single modeoptical fiber into a ferroelectric on insulator waveguide, or into anon-ferroelectric on insulator waveguide, and the second fiber-to-chipcoupler is configured to allow light to be coupled from a ferroelectricon insulator waveguide into a single mode optical fiber, or coupled froma non-ferroelectric on insulator waveguide into a single mode opticalfiber.
 10. The system of claim 6, wherein the multi-layer ferroelectricand non-ferroelectric on insulator waveguides are integrated withelectrodes to implement an optical device, an electro-optical device, ora non-linear optical device, such as an electro-optical modulator, withmicrowave and optical waveguide crossings, and electrical interconnects,compatible with packaging.
 11. The system of claim 6, wherein onephysical optical fiber functions as both the input and the outputoptical fibers, and one physical fiber-to-chip coupler functions as boththe input and output fiber-to-chip couplers.
 12. A method comprising:forming a first insulating layer on a handle wafer; implanting a donorwafer with ions to form a damage layer beneath the surface; bonding thedonor wafer and the handle wafer; annealing the bonded wafers toexfoliate a first film along the ion implant damage layer; forming asecond insulating layer on the first film; implanting a donor wafer withions to form a damage layer beneath the surface; bonding the donor waferto the second insulating layer on the first film; and annealing thebonded wafers to exfoliate a second film along the ion implant damagelayer.
 13. The method of claim 12, wherein the donor wafer is aferroelectric wafer, such as a lithium niobate wafer.
 14. The method ofclaim 12, further comprising patterning the thin film of theferroelectric to form optical waveguides.
 15. The method of claim 12,further comprising pattering electrodes and/or microwave waveguides inproximity to the first and/or second films.
 16. The method of claim 12,wherein at least one of the handle wafer or the donor wafer are die. 17.The method of claim 12, wherein the bow of the handle is matched to thebow of the donor.
 18. The method of claim 12, wherein anon-ferroelectric thin film material is formed on the first or secondinsulating layers by deposition, in lieu of exfoliation, resulting inone ferroelectric thin film and one non-ferroelectric thin film.
 19. Themethod of claim 12, wherein the ferroelectric and non-ferroelectric thinfilms are patterned into optical waveguides.
 20. The method of claim 12,wherein the process is repeated to form multilayer optical waveguideswhere the number of waveguide layers is greater than two.